<xsd:schema
    xmlns:xsd="http://www.w3.org/2001/XMLSchema"
    elementFormDefault="qualified">

<xsd:element name = "plugin">
    <xsd:complexType> <!-- content only! -->
      <xsd:sequence>
        <xsd:element ref = "<error>params</error>" minOccurs = "0" maxOccurs = "1"/>
      </xsd:sequence>
    </xsd:complexType>
  </xsd:element>
  
</xsd:schema>